The invention relates generally to ion implantation, and in particular to improving uniformity of ion beams on wafers during processing.
To better understand the invention, U.S. Pat. Nos. 5,481,116, 4,980,562 and 4,922,106 are incorporated herein by reference as background.
Traditional single-wafer ion implanters either scan the beam across a stationary wafer or translate the wafer in one direction past a fan shaped ion beam or an ion beam scanned in a single axis. The process of scanning or shaping a uniform ion beam generally requires a complex and long beam linexe2x80x94which is undesirable at low energies. Traditional high-current ion implanters achieve a short beam line by placing a large number of wafers on a wheel and simultaneously spinning and radially translating the wheel through the ion beam. The multiple wafer wheel makes an ion implanter undesirably large. It was adopted to reduce heating effects; however that is unnecessary at low energies. There is thus the need to improve wafer scanning systems and methods.
One object of the invention is to provide a substantially uniform dose of ion beam implantation across the surface of a wafer during processing in an ion implanter. Other objects will be apparent in the description that follows.
In one aspect, the invention provides a method of uniformly implanting a wafer with an ion beam. The wafer is generally of the type with a surface area in the form of a disk with a diameter and center. The ion beam is first formed as an elongated shape incident on the wafer, the shape having a length along a first axis smaller than the diameter, and a width shorter than the length along a second axis. Next, the wafer is translated at a variable velocity in a direction substantially parallel with the second axis. The wafer is also rotated substantially about the center at a rotational velocity. These movements are made such that the ion beam implants the wafer with substantially uniform dose across the surface area of the wafer.
In another aspect, the wafer is translated such that the ion beam implants the wafer from one side of the wafer, across the surface area of the wafer, and through another side of the wafer, in a selected velocity versus position profile.
In still another aspect, the step of translating at a variable velocity includes moving the wafer at a greater velocity when the ion beam implants the center of the wafer and at a slower velocity when the ion beam implants an edge of the wafer.
In another aspect, the step of translating includes translating the wafer such that the ion beam implants from one side of the wafer to the center. Preferably, the ion beam is blanked when it reaches the center of the wafer. In addition, the wafer is then preferably declerated in a direction opposite to the scan direction.
The methods of the invention also include, in another aspect, the step of tilting the wafer while rotating the wafer such that the ion beam implants the surface area at a substantially constant angle relative to a crystal axis of the wafer. Preferably, the wafer of this aspect is translated in a direction substantially parallel to the ion beam such that the ion beam implants the surface area with a substantially constant spot size. In another aspect, the wafer is moved in the direction with a magnitude proportional to an impact location of the beam on the wafer relative to a plane perpendicular to the beam that passes through the center.
In yet another aspect, the method of the invention includes the step of determining beam current density of the ion beam. With this determination, the method also preferably includes the step of adjusting the variable velocity as a function of the current density. In another aspect, the method includes the step of adjusting the rotational velocity as a function of the current density.
Current density is preferably measured in two dimensions; and more preferably with a disk having a plurality of holes (and preferably one hole in each angular quadrant of the disk). In accord with the invention, the disk is moved similar to a wafer in translation and rotation while two-dimensional current density is determined. In one aspect, the disk has 4 equally spaced holes made about the disk center, though more holes can be used if desired.
In another aspect, the method of determining current density of the ion beam includes using a Faraday Cup to measure the current density. In one aspect, the disk is removed for direct calibration of the beam into the Cup.
These and other aspects and advantages of the invention are evident in the description which follows and in the accompanying drawings.